Defeat war machine (4)
I believe the answer is:
tank
I believe this clue is a double definition.
'defeat' is the first definition.
(I've seen this before)
'war machine' is the second definition.
(I've seen this before)
(Other definitions for tank that I've seen before include "Armoured vehicle - container for liquid or gas" , "Armour-plated military vehicle" , "Container for liquids - military vehicle" , "Aquarium; army vehicle" , "Cistern - armoured vehicle" .)